"It does seem like it's an expansion of range, rather than a relocation," said John Field, a research fisheries biologist with the National Oceanic and Atmospheric Administration (NOAA) in Santa Cruz, California.

Scientists don't know exactly why this migration is occurring, but they believe the squid are constantly on the prowl for ample supplies of food. Because the animals are so large, an area must be dense with prey to support a stable population.

"I don't think a large number of squid could survive year-round in one place or the other, because they would deplete it," Gilly, the Stanford biologist, said.

Scientists also suspect that global warming may play a part in the migrations.

"These high-turnover animals with really high metabolisms are the kinds of animals that you would expect to respond first and respond more to long-term warming trends," Field, the NOAA scientist, said.

"The fact that this is happening in both hemispheres to me really points to a physical mechanism, a climate-related mechanism."

Voracious Eaters

A fishing industry targeting the jumbo squid has flourished in recent years as the animals' numbers have grown.

Hundreds of skiff fishers depart Santa Rosalia every night during the summer. They use multipronged lures and monofilament lines to haul up squid by hand.

The fishery there processes a hundred thousand tons of squid annually, most of it going to Asian markets.

A huge squid-fishing industry has also been established in central Chile.

But there the squid invasion has led to a decrease in the population of commercially valuable hake fish.

That worries California fishers, who fear the squid invasion may soon cut into the area's fish stocks.

To find out what the squid are eating, Field has for the past two years examined the stomach contents of about 500 animals.

"We see that the most frequently occurring prey item that they eat is Pacific hake, and that's troubling because that's one of the biggest fisheries on the West Coast," he said.

The squid are also chowing down on anchovies, sardines, market squid, and smaller rockfish.

"Those are things that humans catch and exploit and eat," Field said.

In addition, fishers complain that jumbo squid harm their catch by attacking or damaging fish caught on lines or in commercial gear.

Winners and Losers

Field, however, points out that the increasing numbers of squid can have positive consequences as well.

"They're eating a lot of things, but they can also be food for a lot of things," he said. "Some animals are winning, others are probably losing. Who's to say how the final balance will tally out?"

The squid invasion has been a boon to some recreational-fishing businesses, which take clients out for squid fishing in the winter, when there's not much else to reel in.